1. Field of the Invention
This invention relates to a method of driving a diode type display unit wherein display is carried out by means of a combination of a two-terminal element and an electrooptical element.
2. Description of the Prior Art
The term "two-terminal element" used herein means elements, of which the voltage-current characteristics exhibit nonlinearity such as PN junction diode, metal-insulating layer-metal diode (hereinafter referred to simply as "MiM diode") and the like. On the other hand, the term "electrooptical element" means elements, of which the optical quality is controlled by means of impressed voltage such as liquid crystal element, electrochromic element, PLZT element, electroluminescent element, plasma luminescent element, fluorescence luminescent element and the like.
For the sake of simplicity, a MiM diode and a liquid crystal element are utilized as the two-terminal element and the electrooptical element, respectively, in the following description.
FIG. 1 shows a construction of a diode type display unit wherein reference numeral 1 designates an input signal line, i.e., input line of display information, reference numeral 5 designates a display panel part, and this display panel part is one obtained by disposing a unit picture element shown in two dimensional manner in FIG. 2. A scanning electrode line driving circuitry part 3 applies a prescribed voltage to scanning electrode lines of the display panel part. A signal electrode line driving circuitry part 4 applies a prescribed voltage to a signal electrode display panel part 5. A controlling part 2 supplies control signals to the scanning electrode line driving circuitry part 3 and the signal electrode line driving circuitry part 4, respectively, in order to display input information.
In the unit picture element shown in FIG. 2, reference numeral 6 designates a scanning electrode line, 7 a signal electrode line, 8 a MiM diode being a two-terminal element, and 9 a display picture element capacitor composed of a liquid crystal layer being an electrooptical element and a display electrode, respectively.
FIG. 3 illustrates a conventional driving signal waveform wherein scanning electrode signal waveform is represented by solid line whilst signal electrode signal waveform is represented by dotted line. This driving signal waveform consists of two types of periods, i.e., writing periods designated by W and holding periods designated by H in FIG. 3. A pulsing signal 10 or 12 is applied to the scanning electrode line during the writing period W whilst a holding signal 11 or 13 is applied during the holding period H.
On one hand, ON signal 14 or 16 is applied to the signal electrode line in the case when a picture element is in ON display (voltage of display picture element capacitor is high) whilst OFF signal 15 or 17 is applied when the picture element is in OFF display (voltage of the display picture element capacitor is low). Problem of halftone can be solved by setting the voltage signal between OFF and ON signals. During the writing period W, charge is injected into the display picture element capacitor in accordance with display information, and charge of the display picture element capacitor is held by utilizing current-voltage nonlinearity of MiM diode during the holding period H. Since the voltage corresponding to the charge which has been held is continuously applied to the liquid crystal layer, high quality display is possible in comparison with voltage equalization driving method which exhibits remarkable deterioration in display quality due to increase of number of scanning electrodes.
The problem of such conventional driving method composed of the writing and holding periods as mentioned above resides in that the charge of the display picture element capacitor immediately after the writing period depends upon the charge which has been written in the preceding writing period to the aforesaid writing period. In this connection, the problem will be more specifically described by referring to FIG. 4 wherein reference character W designates a writing period, and H.sub.1, H.sub.2 holding periods before and after the writing period, respectively. In FIG. 4, voltage across both ends of the display picture element capacitor 9 is plotted as ordinate and time as abscissa wherein reference numerals 18 and 19 designate voltages across both the ends of the display picture element capacitor 9 in case of OFF display and ON display during the holding period H.sub.1, respectively, numeral 22 designates a voltage during the holding period H.sub.2 when the charge corresponding to ON display was written during the writing period W, and numerals 20, 21 designate voltages when OFF displays were written, respectively. When ON display was written, the voltage after writing becomes the situation 22 in either case that display is ON 18 or OFF 19 during the holding period H.sub.1. As a result, the ON display voltage 22 is obtained, which does not depend on the display situation prior to the writing period. On the other hand, when OFF display was written during the writing period W, the situation 18 becomes the voltage 20 in case of OFF display during the holding period H.sub.1 whilst the situation 19 becomes the voltage 21 in case of ON display during the holding period H.sub.2. In other words, the voltages in case of OFF display during the holding period H.sub.2 depend upon the display situation before writing period as represented by reference numerals 20 and 21. Such dependence results in decrease in display quality such as display reliability, contrast ratio, and the like.